As Musky says. With VAT/TVA well over £300.
But a reference point that is much more sensible would be Andrew at Priory Magnetos once again. Where a rebuilt unit can be had for £120 when available, or a unit can be reconditioned for round about the same, and tired units are taken in at up to £40 in part exchange. Fibre or alloy pinions available, fibre recommended by him. (Silent running, potentially sacrificial in event of a lock-up, etc).
In the past, I've regarded usable but not reconditioned units as worth between 50€ and 80€, or up to 110€-ish (tops) if I've fitted a new fibre gear.
Whether there is any premium attached to a steel gear (sold attached or sold separately), I doubt. And yes, it can be swapped for a fibre one, but need to check that the big circular shim that goes between the pinion and the mechanism is there, as it might have been omitted with a metal gear fitted.
Can't see a lot out there on e-bay etc at the moment (except some replica parts from the usual suspects), but haven't looked too hard. There was one yesterday at around £75 from memory.